Excited-State Switching Frustrates the Tuning of Properties in Triphenylamine-Donor-Ligand Rhenium(I) and Platinum(II) Complexes

The photophysical properties of a series of rhenium(I) tricarbonyl and platinum(II) bis(acetylide) complexes containing a triphenylamine (TPA)-substituted 1,10-phenanthroline ligand have been examined. The complexes possess both metal-to-ligand charge-transfer (MLCT) and intraligand charge-transfer (ILCT) transitions that absorb in the visible region. The relative energies and ordering of the absorbing CT states have been successfully controlled by changing the metal center and modulating the donating ability of the TPA group through the addition of electron-donating methoxy and electron-withdrawing cyano groups. The ground-state properties behave in a predictable manner as a function of the TPA substituent and are characterized with a suite of techniques including electronic absorption spectroscopy, resonance Raman spectroscopy, electrochemistry, and time-dependent density functional theory calculations. However, systematic control over the ground-state properties of the complexes does not extend to their excited-state behavior. Unexpectedly, despite variation of both the MLCT and ILCT state energies, all of the luminescent complexes displayed near-isoenergetic emission at 298 K, yet the emissive lifetimes of the complexes vary from 290 ns to 3.9 μs. Excited-state techniques including transient absorption and transient resonance Raman, combined with a suite of quantum-chemical calculations, including scalar relativistic effects to elucidate competitive excited-state relaxation pathways, have been utilized to aid in assignment of the long-lived state in the complexes, which was shown to possess differing ³MLCT and ³ILCT contributions across the series.